Key component for wireless communication with terahertz frequencies

The THz wave (green) and the laser light (red) are both split in half by the beam splitter (grey plane), providing the necessary phase shift of the waves. The laser light is mixed with THz radiation in special crystals (brown planes), and subsequently two sidebands (blue waves) are generated. Both laser light modulations are then coupled in the grey cylinders in optic fiber (tan wires) and combined in the multimode interference structure (white MMI plane). The result is that one sideband extinguishes and that the intensity of the other sideband is maximixed, solving the problem of THz signal distortion in the optic fiber network.

Credit: Image courtesy of Radboud University

https://www.sciencedaily.com/releases/2016/10/161004085303.htm

An ultrahigh speed, wireless communication network using THz instead of GHz frequencies is now one step closer. Researchers at Radboud University's FELIX Laboratory have shown that it is possible to effectively transmit signal waves with THz frequencies through the existing fiber optic network.

HD television, big data, the internet of things and social media have considerably increased the data rate of our wireless communication network, and continue to do so. An obvious way to facilitate this network growth is to use terahertz frequencies (THz, 1012 Hertz) with high-speed data rates of up to 100 Gbit/s. Current wireless data communication systems operate at an average speed of 100Mbi/s using microwave frequencies around one gigahertz (GHz, 109 Hertz). For instance: GPS systems work with 1,3 GHz frequencies, wifi with 2,4 and 5 GHz, and your microwave with 2,45 GHz. In the search for free frequencies, the unexplored THz area is of great interest.

Distortion of terahertz signals

For wireless THz surfing on the Internet, it is necessary to connect THz wireless stations to the worldwide fiber optic network. However, existing microwave techniques do not operate at THz frequencies. "THz is a difficult frequency region, because it is both electronic and optic at the same time," FELIX researcher Giel Berden explains. "It is too low for normal optics, and too high for standard electronics." Moreover, THz signal waves in the fiber optic network are scrambled, because standard modulation of laser light generates two sidebands (colours) that interfere with one another. Optical Single Side Band (OSSB) is a method to prevent this scrambling of information by selectively extinguishing one sideband.

Special beam splitter

Scientists at Radboud University's FELIX Laboratory developed an OSSB modulator that enables wireless THz waves to be transmitted unperturbed through the fiber network. First author Afric Meijer explains: "With a specially designed beam splitter that splits both the THz waves and the infrared laser light in half, one of the two sidebands is reduced by a factor of over sixty, while the other sideband's intensity increases significantly." The special modulator does not contain any moving parts or colour filters, and operates over an ultra-wide bandwidth from 0.3 to 1 THz.

The THz OSSB modulator is a by-product of the research by TeraOptronics on the THz laser FLARE (Free-electron Laser for Advanced spectroscopy and high-Resolution Experiments) at Radboud University. "The apparatus to determine the colour of FLARE's laser light was exactly what was needed to observe THz OSSB," Meijer explains. "Both the special THz laser FLARE and Afric's interest to expand communication with THz frequencies were imperative to make an impact in this field that was new to us," says co-author Wim van de Zande, currently Director of Research at ASML.

Opportunities for ultra HD, virtual reality and big data

As THz signals in the air are strongly absorbed by water vapour, wireless THz communication will mostly be used for relatively short distances. Meijer: "Our THz OSSB modulator allows us to use the existing fiber optic network. Ultra HD and Virtual Reality images can be received or transmitted wirelessly through a THz link, just like the petabytes of data in research institutes and hospitals." Berden: "This publication is a proof of principle. To actually use the technique requires a couple of additional steps, for instance scaling down the design for microfabrication and improvements in efficiency. Our hope is that this idea will be further developed by the industry."

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Story Source:

Materials provided by Radboud University. Note: Content may be edited for style and length.

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Journal Reference:

1. A. S. Meijer, G. Berden, D. D. Arslanov, M. Ozerov, R. T. Jongma, W. J. van der Zande. An ultrawide-bandwidth single-sideband modulator for terahertz frequencies.Nature Photonics, 2016; DOI: 10.1038/nphoton.2016.182

New route for switching magnets using light

Impression of an iron oxide crystal lattice. Red: spins of iron ions, Blue: oxygen ions. Green: electrons in their orbit responsible for the exchange interaction. The interaction keeps the spins aligned. A light pulse excites the electrons, changes the exchange interaction and thus releases the spins.
 http://phys.org/news/2015-09-route-magnets.html

An international team led by Radboud University physicists has discovered that reversing the poles of magnets must be possible without a heating or a magnetic field.. A strong pulse of light can have a direct effect on the strong quantum mechanical 'exchange interaction', therefore changing the magnetism (Nature Communications, 16 September 2015).

In 2007, Professor Rasing and his group at Radboud University showed for the first time that fast pulses of laser light can reverse the poles of magnets. This was a paradigm shift as, until then, physicists believed that light could never be strong enough to break the strong magnetic interaction forces. It can, however, and very local heating by the laser pulse in combination with differences in the response times of the constituent atoms can explain this phenomenon. The researchers have now discovered a new way in which light can manipulate magnetisation.

Directly on the electrons

In the article published by Nature Communications on September 16 the researchers show that the light can excite electrons, which in turn can directly influence the strength of the exchange interaction and therefore change the magnetisation. No heat is released in the process, which is good news for magnetic data storage applications as it means that the method requires little energy. Exchange interaction refers to the internal, quantum mechanical forces that make a magnet magnetic.

"We carried out our experiments in iron oxides, including hematite," says project leader Alexey Kimel. "The crystal structure of hematite is a good system to study this mechanism, as the iron ions are neatly separated by oxygen ions in the crystal lattice. Even so, exchange interaction takes place between the iron ions because the electrons interact through the oxygen ions. By exciting the electrons in the oxygen with a pulse of light, we can manipulate the exchange interaction between the iron spins, and perhaps even reverse their polarity in the near future."

Cool savings

Switching with no heat has the potential to revolutionise magnetic data storage. Huge amounts of heat are currently released in large data centres, and good cooling is becoming a big problem. Facebook, for example, is planning to build its new data centre in the north of Sweden for this very reason. "If we can store information using a new, cool method, data storage will be a lot cheaper," explains Kimel.

Measure what you do

The researchers also developed a magnetometer to measure the ultrafast changes they induce in a magnet. They use the freely propagating electromagnetic radiation in the Terahertz frequency range (1 THz = 1012 Hz) emitted by the spins of the magnet. By measuring the changes in this radiation, they are able to measure the effect of light on the magnetisation. "We have produced a magnetometer that measures at the femtosecond scale," says Rostislav Mikhaylovskiy, the first author of the article.

To be continued at FELIX and HFML

The researchers will conduct further studies into switching using light in the new FELIX laser lab and the adjacent HFML in Nijmegen. The strength of magnetic fields generated by HFML is comparable to that of the exchange interaction and the frequency of light waves generated by FELIX can be tuned to affect the electrons and change the strength of the exchange interaction in the most effective way. "This will certainly help us explore this mechanism in greater detail," says Theo Rasing.

 Explore further: Magnetization can surf on the top of a laser-induced sound wave

More information: "Ultrafast optical modification of exchange interactions in iron oxides." Nature Communications. 16 September 2015 DOI: 10.1038/ncomms9190

The FELIX laboratory at Radboud University

http://www.ru.nl/english/research/radboud/themes/physics/vm/laser-laboratory/

The FELIX laboratory at Radboud University houses the first laser in the world which can produce wavelengths in the far infrared region – waves up to 1.5 millimetres. How do biomaterials and other materials react to these long light waves? And what can we learn from this? The equipment in the FELIX lab reveals new characteristics of materials.

FELIX’s three lasers each produce their own range of wavelengths. Together, they provide the world’s largest tuning range in the infrared and terahertz spectrum.

The three different lasers are:

• FELIX: Free-electron Laser for Infrared eXperiments
• FELICE: Free-electron Laser for IntraCavity Experiments
• FLARE: Free-electron Laser for Advanced spectroscopy and high-Resolution Experiments

The infrared radiation of the FELIX lasers interacts with molecules and materials. This can reveal detailed information about 3D structure and electronic properties. In this way, FELIX and its users help to explore the secrets of nature and to develop new technologies. For instance: how our body converts food into energy, how to build a quantum computer or how our planet was once formed from stardust.

FELIX’s neighbour, the high-field magnet laboratory HFML, hosts some of the most powerful magnets in the world and is another unique facility at Radboud University. The combination of these magnets with the FELIX lasers offers scientists possibility to study matter and materials in conditions that cannot be found anywhere else in the world.

Technical challenges
In November 2012 researchers from FELIX and HFML performed a joint experiment for the first time. HFML researcher Hans Engelkamp: 'It’s difficult to transport light with a wavelength of around one millimetre without losing a lot of energy. Light with such long waves fans out quickly and is therefore hard to direct. In this experiment we focused the light in a brass tube, which acts as a mirror. However, every reflection costs energy, so the challenge is to minimise the energy loss.'

• Quantum computer a step closer with help from FELIX lab (Maart 2015)
• Preview of the new laser lab FELIX in July 2011 (in Dutch)
• Reportage of the new FLARE laser in October 2011 (in Dutch)

FELIX is ran by researchers from the Molecular and Biophysics group of the Institute for Molecules and Materials at Radboud University Nijmegen. The FELIX and FELICE lasers originally came from the laboratory of the FOM Institute for Plasma Physics Rijnhuizen in the Netherlands. The FLARE lab is part of the Nijmegen Center for Advanced Spectroscopy (NCAS).